1. Field of the Invention
This invention relates to a liquid-jet head, and a liquid-jet apparatus, where a portion of a pressure generating chamber communicating with a nozzle orifice for ejecting a liquid is constituted of a vibration plate, a piezoelectric element is formed on the surface of the vibration plate, and the liquid is ejected by displacement of the piezoelectric element.
2. Description of the Prior Art
An example of a liquid-jet apparatus is an ink-jet recording apparatus having an ink-jet recording head equipped with a plurality of pressure generating chambers for generating pressure for ink droplet ejection by a piezoelectric element or a heating element; a common reservoir for supplying ink to the respective pressure generating chambers; and nozzle orifices communicating with the respective pressure generating chambers. This ink-jet recording apparatus applies ejection energy to ink within the pressure generating chamber communicating with a nozzle corresponding to a printing signal to eject ink droplets through the nozzle orifice.
Such an ink-jet recording head is roughly classified into two types. One of them is a recording head in which a heating element, such as a resistance wire, for generating Joule heat in response to a drive signal is provided within a pressure generating chamber, as stated above, and ink droplets are ejected through a nozzle orifice by bubbles produced by the heating element. The other recording head is that of a piezoelectric vibration type in which a portion of a pressure generating chamber is constituted of a vibration plate, and the vibration plate is deformed by a piezoelectric element to eject ink droplets through a nozzle orifice.
Two types of the ink-jet recording head under the piezoelectric vibration system have found practical use, namely, a recording head using a piezoelectric actuator of longitudinal vibration mode which expands and contracts the piezoelectric element in the axial direction, and a recording head using a piezoelectric actuator of flexural vibration mode.
The former recording head can change the volume of the pressure generating chamber by abutting the end surface of the piezoelectric element against the vibration plate, and enables manufacturing of a head suitable for high density printing. However, this recording head needs a difficult step of cutting and dividing the piezoelectric element in a comb tooth shape in conformity with the array pitch of the nozzle orifices, and also requires an operation for aligning and fixing the divisions of the piezoelectric element to the pressure generating chambers. Consequently, the manufacturing process is complicated.
In the latter recording head, on the other hand, the piezoelectric element can be fabricated and installed on a vibration plate by a relatively simple step of adhering a green sheet of a piezoelectric material to the shape of the pressure generating chamber, and then sintering the green sheet. However, a certain size of the vibration plate is required because of the usage of flexural vibration, thus posing difficulty in achieving a high density array of the piezoelectric elements.
To resolve the disadvantage of the latter recording head, a recording head, as shown in Japanese Unexamined Patent Publication No. 1993-286131, is proposed, in which a uniform piezoelectric material layer is formed throughout the surface of the vibration plate by a deposition technology, and the piezoelectric material layer is cut and divided into a shape corresponding to the pressure generating chamber by a lithography method, so that piezoelectric elements are formed independently of each other for the respective pressure generating chambers.
According to the above-described process, an operation for adhering the piezoelectric element onto the vibration plate is unnecessary. The advantage is also conferred that not only the piezoelectric elements can be fabricated and installed with high density by lithography, which is an accurate and simple method, but also the thickness of the piezoelectric element can be decreased to permit a high-speed drive.
The piezoelectric element is formed, for example, by stacking a lower electrode, a piezoelectric layer, and an upper electrode in this order on one surface of a single crystal silicon substrate. The piezoelectric layer is generally a polycrystalline thin film composed of lead zirconate titanate (PZT) or the like, and has a columnar growth structure where many interfaces among the crystals, namely, many grain boundaries, are present.
With the above-described ink-jet recording head, for example, a drive voltage is applied from external wiring or the like to the lower electrode and the upper electrode having the piezoelectric layer sandwiched therebetween to generate a predetermined drive electric field in the piezoelectric layer, thereby causing flexural deformation to the piezoelectric element and the vibration plate. As a result, the internal pressure of the pressure generating chamber is substantially raised to eject ink droplets from the nozzle orifice.
Such a conventional ink-jet recording head has many grain boundaries existent between the crystals of the piezoelectric layer. These grain boundaries constitute the cause of hampering the expansion and contraction of the piezoelectric layer, i.e., the expansion and contraction of the columnar crystals. Thus, the amount of displacement of the piezoelectric element cannot be set at a predetermined value. This poses the problem that ink ejection cannot be performed at maximum output, namely, with maximum amount of displacement of the piezoelectric element when a certain driving electric field is generated in the piezoelectric layer. Even when the predetermined driving electric field is generated in the piezoelectric layer, the problem arises that under the influence of the grain boundaries, the piezoelectric characteristics of the piezoelectric element substantially fluctuate.
These problems are not limited to the ink-jet recording head, but needless to say, occur similarly in other liquid-jet heads.